Everything is happening inside Ouka's head as her body heads towards death(or not since the story explicitly said the "Suicide Parabellum" won't kill the victim, so maybe just until brain death). In the mean time Chihaya is chasing her consciousness hoping to get her back.
However, for every level we enter, exists it's own Chihaya. We know that mafia Chihaya (ch. 1) is the lvl 8 Chihaya as Ouka welcomes her to lvl 9, where we meet school girl Chihaya. It's uncertain which lvl our (idol) Chihaya is from as we see the first Hooded Chihaya go "upstairs" after their encounter (ch. 3).
At some point we find that this 'game' has been going on a lot longer than we though (?) since some Chihaya's have been here so long they've lost their minds, either jumping through lvls or simply spending time on certain lvls. We also learn that the deeper we go the slower time passes by comparison to the last world all the way up to reality.
Every time the world is about to end, 'Ouka' kills her current self to stall for more time as her consciousness can continue to survive, similair to Zeno's Dichotomy paradox where she come infinitesimally close to the end. It's likely that due to this, Chihaya's from the lower levels may have lived for hundreds if not thousands of years going up and down Ouka's consciousness.
Example. In the time that out Idol Chihaya saw Ouka kill herself, to the time where she jumped in, although only seconds have passed for her, days have passed on the other side of the hole, where another hole may have opened. Eventually translating to an near infinite amount of Chihaya's suddenly existing, each exponentially older than the last.
Back to the mansion, essentially two factions meet. Jaded Chihaya's that climbed up from the depths who have lived long enough to understand the truth of the world they exist in, and hopefull Chiyaha's who are still looking to save Ouka. The jaded Chihaya's know that if Ouka is saved then infinite worlds will be destroyed so they stop the young Chihaya's from saving Ouka and bringing her back to reality which will end this entire dream. So they kill the young Chihaya's.
The three then kill themselves simply because they want out, their tired, they've been at least from their point of view, saving the multiverse for the last god know's how long. They see that out (idol) Chihaya understand the situation and decided to leave it in her hands, which proves to be the right decision? (ch. 9 last page)
Lastly, it's likely that the Ouka we see is only part of her if we follow the old id, ego, superego model of the human physche.
So now our Chihaya is just gonna keep killing all the other Chihayas? Until when though? Also, I wonder is the crazy killer Chihaya still out there? If so, did she have the same idea of killing all tje other Chihaya's but at some point, bexame insane and decided to kill Ouka as well? I wish I had big brain now